This invention relates to a wide-band hybrid network, which employs a wide-band, two-wire transmission line and enables simultaneous long-distance transmission of a wide-band signal in two directions over the entire transmission frequency band of the transmission line.
A conventional wide-band, two-way transmission system of the two-wire type for long-distance transmission with a large capacity is a frequency division transmission system, in which in case of an analog signal such as an FDM-AM (frequency division multiplex-amplitude modulation) signal, the entire frequency band of the transmission system is divided into two frequency bands, so that the divided bands are used for different transmission directions. However, this system is defective in that the band used for each direction is half the entire frequency band and in that expensive directional filters must be used for terminal equipments and respective repeaters. Further, there has also been proposed a two-wire, two-way simultaneous transmission system utilizing the entire frequency band of the transmission system by the employment of hybrid circuits. In this case, however, the leakage characteristic of the hybrid circuit has the following technical problems:
I. THE BALANCE OF THE HYBRID CIRCUIT ITSELF, AND
II. THE IMPEDANCE MIS-MATCHING BETWEEN THE HYBRID CIRCUIT AND THE TWO-WIRE TRANSMISSION LINE.
Moreover, such a hybrid circuit as herebefore employed for two-wire to four-wire conversion for an analog signal has a very narrow frequency band, so that the leakage due to the incompleteness of balance of the hybrid circuit itself mentioned at the item (i) does not cause any trouble but the leakage caused by the impedance mis-matching between the hybrid circuit and the two-wire transmission line and reflection due to impedance mismatching in the two-wire transmission line and terminal equipments present a problem as mentioned in item (ii). On the other hand, in such wide-band transmission system as the type to which this invention is applied, a high quality transmission line such as a coaxial cable is employed, so that (i) the balance of the hybrid circuit itself is more important than the problem (ii). It is appreciably difficult to realize a highly balanced hybrid circuit over a wide band. In the case of using conventional hybrid circuits, the intervals of repeaters become short due to leakage so that the application of such conventional hybrid circuits to a long-distance transmission system is difficult.
In a two-wire, simultaneous two-way transmission system for transmitting PCM base band signals, it is impossible to apply such frequency division transmission technique to this system, as employed in an analog transmission system, so that a system employing hybrid circuits is taken into consideration. However, a two-way transmission system employing hybrid circuits encounters the problem of leakage as does the case of the analog transmission and, therefore, it is very difficult to realize such an intended system by the use of the conventional hybrid circuits.